1. Field of the Invention
This invention relates to a method for smoothing a rough surface of a silicon single crystal substrate to obtain a highly flat and smooth surface.
2. Description of the Related Art
A silicon single crystal substrate has a mirror surface polished, in general, by a chemical-mechanical polishing (CMP) method. This is one of the conditions required when it is desired to form a very fine structure on the surface of the silicon single crystal substrate in a step of fabricating an electronic device and so on, in order to obtain a high manufacturing yield.
For example, the design rules of electronic device are almost in the stage of requiring 0.3 to 0.2 .mu.m of deep submicron order of accuracy as a practical level. For fabrication of such an electronic device, some of thin films formed on a silicon single crystal substrate are required to have a thickness as thin as several nm for a gate insulating film, etc. That is, a microscopic smoothness of the surface of the substrate has great influences even on the quality of the thin film.
In order to prevent adverse influences of micro-defects in the substrate to device characteristics, there has been in these years employed a so-called epitaxial wafer in which a silicon single crystal thin film is epitaxially grown on a silicon single crystal substrate. Sufficient removal of foreign matter such as a native oxide film or particles from the substrate prior to the epitaxial growth is essential from the viewpoint of avoiding defects in a grown layer. In addition to it, the smoothness of the substrate surface is one of highly important factors. A mean squared surface roughness Rms of a mirror surface attained by the above-mentioned CMP is about 0.1 nm at most at the current stage. However, the surface roughness will be required to be made much smaller as the design rules of electronic device demand severer accuracy requirements in future.
Meanwhile, as the silicon single crystal substrate for use in fabrication of the aforementioned epitaxial wafer, there has been frequently employed a p type substrate which contains boron (B) atoms with a high concentration of the order of 10.sup.18 /cm.sup.3 or more. This is because the substrate has a low resistivity of 0.01 to 0.02 .OMEGA..multidot.cm, a large mechanical strength and an excellent gettering efficiency. The p type low-resistivity substrate however is inferior to a high-resistivity substrate having a resistivity of 10 .OMEGA..multidot.cm (and having an impurity concentration of the order of 10.sup.16 /cm.sup.3 or less), in microscopic smoothness. That is, its mean squared surface roughness Rms is close to 0.2 nm.
In order to improve the microscopic smoothness of such a p type low-resistivity substrate, there has been proposed a method for performing heat treatment on a substrate at a high temperature of 1100.degree. C. or higher in a hydrogen (H.sub.2) gas atmosphere. And this method is already put in industrical practical use.
However, this method has a problem that, since boron atoms doped in the silicon single crystal substrate are vaporized and emitted into the atmosphere at such a high temperature, a distribution of the substrate resistivity varies in a depth direction.
The vaporization of boron atoms can be suppressed to a large extent when the thermal treatment temperature is lowered to 1000.degree. C. or lower. Details for it are disclosed in a Journal of the Electrochemical Society vol. 143, p. 677, 1996.
However, in the case where the thermal treatment temperatue to the silicon single crystal substrate is reduced, only when all the temperatures of not only the above H.sub.2 gas processing but also all the other processes can be reduced, the boron atom dissipation can be suppressed. A related problem is the temperature of pretreatment for removal of the native oxide film or deposited organic particles.
A method usually employed to remove the native oxide film is to perform thermal treatment on the substrate at a high temperature of 1100.degree. C. or so in an atmosphere of an H.sub.2 gas or HCl/H.sub.2 mixture gas. Known as other methods for removing the native oxide film at a room temperature, are a wet etching process using a dilute hydrofluoric acid, a process using a combination of a hydrogen fluoride gas and steam, and an Ar plasma process. These processes however have problems that an oxide film re-grows immediately after the treatment, the surface of the substrate become rough, processing facilities are subject to corrosion, etc. At the current stage, the former high-temperature treatment is considered the best.
Meanwhile, organic substances are floating in the air of a clean room installed in a semiconductor plant. The inventors of the present application have found a new fact from experiments that, when a silicon single crystal substrate is left to stand within such a clean room for, e.g., one day, organic particles floating in the room air deposite on the substrate to form an organic substance film of a thickness corresponding to 0.1 to 0.2 nm of silicon oxide film.
This deposited organic film can also be removed typically by a similar high temperature heat treatment to the above in an atmosphere of an H.sub.2 gas or a mixture gas of H.sub.2 and HCl. As other methods for removing the organic film at a room temperature, there may be considered an ozone(O.sub.3) treatment, an ultraviolet (UV) irradiation treatment and a treatment corresponding to a combination thereof. These methods, however, are not preferable because an oxide film eventually re-grows on a substrate.
In all of these prior art pretreatment methods, when the treatment temperature is reduced to 1000.degree. C. or lower, it becomes difficult to sufficiently remove the native oxide film or deposited organic particles.
When a silicon single crystal substrate with a surface film such as a native oxide film treated for smoothing with use of a high-temperature H.sub.2 gas, there occurs a problem that micro-holes are formed on the surface of the silicon single crystal substrate, which is explained in a Journal of the Electrochemical Society, vol. 142, p. 3092, 1995. This is because an etching rate of the H.sub.2 gas to silicon (Si) is far larger than that of the H.sub.2 gas to silicon oxide (SiO.sub.x) such as a native oxide film, which results in that the etching of the underlying silicon single crystal substrate advances from iside of pinholes or a part of a native oxide film where its thickness is locally thinner. With it, not only microscopic smoothing but also macroscopic smoothing cannot be realized.
In this way, the prior art methods have difficulties in sufficiently removing the native oxide film and deposited organic particles by way of reducing the treatment temperature to lower than 1000.degree. C. and simultaneously realizing the microscopic smoothing of the surface of the silicon single crystal substrate. This has led to a cause of giving limitations to manufacturing of the epitaxial wafer with use of the p type low-resistivity substrate.